In a typical cellular radio system, user equipment units (also referred to as UEs, wireless terminals, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node (also referred to as a base station, a network node, a “NodeB”, and/or enhanced NodeB “eNodeB” or “eNB”).
Dynamic control of the UE transmission power is a common feature in cellular systems. The objectives of uplink power control include: (a) reaching a sufficient received power and signal quality on the used channel at the serving base station, (b) limiting the received power (interference) at non-serving base stations, (c) limiting the received power (interference) on non-used channels at the serving base station and (d) reducing the output power level to limit power consumption and save battery life in the UE.
Power control schemes can further be divided in to the categories “closed-loop” and “open-loop” depending on what type of measurement input is used. Closed-loop schemes make use of measurements on the same link direction that the power control applies to, i.e. on the uplink for uplink closed loop power control. Open-loop schemes make use of measurements on the opposite link direction, i.e. on the downlink for uplink open-loop power control. In comparison, closed-loop schemes are typically more accurate than open-loop schemes, but also require more control signaling overhead.
Improved support for heterogeneous network operations is part of the ongoing specification of 3GPP LTE Release-10, and further improvements are discussed in the context of new features for Release-11. In heterogeneous networks, a mixture of cells of differently sized and overlapping coverage areas are deployed. One example of such deployments is illustrated in FIG. 1 where pico cells 110 are provided by pico eNBs 112 deployed within the coverage area of a macro cell 100 provided by a macro eNB 102. Other examples of low power radio network nodes, also referred to as points, in heterogeneous networks are home base stations and relays. As will be further discussed below, the large difference in output power (e.g. 46 dBm in macro cells 100 and 30 dBm or less in pico cells 110) results in a different interference situation than what is seen in networks where all base stations have the same output power.
The aim of deploying low power nodes such as pico base stations 112 within the macro cell coverage area 100 is to improve system capacity by means of cell splitting gains as well as to provide users with wide area experience of very high speed data access throughout the network. Heterogeneous deployments can be particularly effective to cover traffic hotspots, i.e. small geographical areas with high user densities served by e.g. pico cells 110, and they represent an alternative deployment to denser macro networks.
One basic approach for operating heterogeneous networks is to provide frequency separation between the different layers. For example, the macro cell 100 and pico cell 110 in FIG. 1 can be configured to operate on different non-overlapping carrier frequencies and thereby avoid interference between the layers. With no macro cell interference towards the under laid cells (here exemplified by the pico cells 110 in FIG. 1), cell splitting gains are achieved when all resources can simultaneously be used by the under laid cells. The drawback of operating layers on different carrier frequencies is that it may lead to resource-utilization inefficiency. For example, with low activities in the pico cells 110, it could be more efficient to use all carrier frequencies in the macro cell 100 and then basically switch off the pico eNBs 112 use of certain frequencies to avoid interference. However, the split of carrier frequencies across layers is typically done in a static manner.
Another (related) approach for operating heterogeneous networks is to share radio resources on the same carrier frequencies by coordinating transmissions across macro and under laid cells. This type of coordination refers to as inter-cell interference coordination (ICIC) in which certain radio resources are allocated for the macro cells 100 during some time period whereas the remaining resources can be accessed by the under laid cells (e.g., pico cells 110) without interference from the macro cell 100. Depending on the traffic situations across the layers, this resource split can change over time to accommodate different traffic demands. In contrast to the above split of carrier frequencies, this way of sharing radio resources across layers can be made more or less dynamic depending on the implementation of the interface between the nodes. In LTE, an X2 interface has been specified in order to exchange different types of information between base stations. One example of such information exchange is that a base station can inform other base stations that it will reduce its transmit power on certain resources.
Time synchronization between base stations is required to ensure that ICIC across layers will work efficiently in heterogeneous networks. Synchronization can be particularly important for time domain based ICIC schemes where resources are shared in time on the same carrier.
LTE uses OFDM in the downlink and DFT-spread OFDM in the uplink. The basic LTE physical resource can thus be represented as a time-frequency grid of radio interface resources as illustrated in FIG. 2, where each resource element corresponds to one subcarrier during one OFDM symbol interval (on a particular antenna port).
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of 1 ms as illustrated in FIG. 3. A subframe is divided into two slots, each of 0.5 ms time duration.
Resource allocation in LTE is defined in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two in time consecutive resource blocks represent a resource block pair and corresponds to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe where the base station transmits downlink assignments/uplink grants to certain UEs via the physical downlink control channel (PDCCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and spans (more or less) the whole system bandwidth. A UE that has decoded a downlink assignment, carried by a PDCCH, knows which resource elements in the subframe that contain data aimed for the UE. Similarly, upon receiving an uplink grant, the UE knows which time/frequency resources it should transmit upon. In LTE downlink, data is carried by the physical downlink shared channel (PDSCH) and in the uplink the corresponding channel is referred to as the physical uplink shared channel (PDSCH).
Demodulation of sent data requires estimation of the radio channel which is done by using transmitted reference symbols (RS), i.e. symbols known by the receiver. In LTE, cell specific reference symbols (CRS) are transmitted in all downlink subframes and, in addition to assist downlink channel estimation, they are also used for mobility measurements and for uplink power control performed by the UEs. LTE also supports UE specific RS aimed only for assisting channel estimation for demodulation purposes.
FIG. 4 illustrates how the mapping of physical control/data channels and signals can be done on resource elements within a downlink subframe. In this example, the PDCCHs occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data could start already at the second OFDM symbol. Since the CRS is common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to UE specific RS which means that each UE has RS of its own placed in the data region of FIG. 4 as part of PDSCH.
The length of the control region, which can vary on subframe basis, is conveyed in the Physical Control Format Indicator CHannel (PCFICH). The PCFICH is transmitted within a control region, at locations known by UEs. After a UE has decoded the PCFICH, it thus knows the size of the control region and in which OFDM symbol the data transmission starts.
Also transmitted in the control region is the Physical Hybrid-ARQ Indicator Channel. This channel carries ACK/NACK responses to a UE to inform whether the uplink data transmission in a previous subframe was successfully decoded by the base station or not.
While PUSCH carries data in the uplink, PUCCH is used for communicating control. PUCCH is a narrowband channel using an RB pair where the two RBs are on opposite sides of the potential scheduling bandwidth. PUCCH is used for conveying ACK/NACKs, periodic CSI feedback, and scheduling request to the network.
Before an LTE UE can communicate with an LTE network it first has to find and acquire synchronization to a cell within the network, i.e. performing cell search. Then the UE receives and decodes system information needed to communicate with and operate properly within the cell, and finally the UE accesses the cell using a so-called random-access procedure.
In order to support mobility, a UE needs to continuously search for, synchronize to, and estimate the reception quality of both its serving cell and neighbor cells. The reception quality of the neighbor cells, in relation to the reception quality of the current cell, is then evaluated in order to conclude if a handover (for UEs in connected mode) or cell re-selection (for UEs in idle mode) should be carried out. For UEs in connected mode, the handover decision is taken by the network based on measurement reports provided by the UEs. Examples of such reports are reference signal received power (RSRP) and reference signal received quality (RSRQ). A UE uses these measurements, and may further use a configurable offset, to be connected to the cell with the strongest received power, or the cell with the best path gain, or something between the two. These selection strategies do not result in the same selected cell as the base station output powers of cells of different type are different. This is sometimes referred to as link imbalance. For example, the output power of a pico base station or a relay is in the order of 30 dBm or less, while a macro base station can have an output power of 46 dBm. Consequently, even in the proximity of the pico cell, the downlink signal strength from the macro cell can be larger than that of the pico cell. From a downlink perspective, it is often better to select a cell based on downlink received power, whereas from an uplink perspective, it would be better to select a cell based on the path loss. Various cell selection approaches are illustrated in FIG. 5 for a pico eNB 112 that provides a pico cell that is within a macro cell 100 provided by a macro eNB 102.
Referring to FIG. 5, in the above scenario, it may be a better case from a system perspective to connect to the pico eNB 112 even if the macro eNB 102 downlink is much stronger than the pico eNB 112 downlink. However, ICIC across layers would be needed when UEs operate within the region between an UL border 502 and a DL border 500 (the link imbalance zone) depicted in FIG. 5. Some form of interference coordination across the cell layers is especially important for the downlink control signaling. If this interference situation is not handled appropriately, a UE in the region between the UL border 502 and the DL border 500 in FIG. 5 and connected to the pico eNB 112 cannot receive the downlink control signaling from the pico eNB 112.
One approach of providing ICIC signaling across layers is illustrated in FIG. 6, where an interfering macro eNB (downlink interference towards a pico eNB) avoids scheduling unicast traffic in certain subframes, implying that neither PDCCHs nor PDSCH occur in those subframes. In this approach, it is possible to create low interference subframes, which can be used to protect UEs connected to pico ENBs and operating in the link imbalance zone. The macro eNB indicates via the backhaul interface X2 to the pico eNB which subframes it will avoid scheduling UEs within. The pico eNB can then take this information into account when scheduling users operating within the link imbalance zone; such that these UEs are scheduled in subframes aligned with the low interference subframes at the macro layer, i.e. in interference protected subframes. However, UEs connected to a pico ENB and operating within the DL border can be scheduled in all subframes, i.e. in both protected and non-protected subframes.
In principle, data transmission (but not control signaling) in different layers could also be separated in the frequency domain by ensuring that scheduling decisions in the two cell layers are non-overlapping in the frequency domain, e.g. by exchanging coordination messages between the different base stations. LTE specifications do not allow control signaling to span the full bandwidth and hence, a time-domain approach must be used.
Classical versus Single Cell Deployments
One prior art approach for deploying a network is to let different transmission/reception radio network nodes form separate cells. That is, the signals transmitted from or received at a radio network node are associated with a cell-id that is different from the cell-id employed for other nearby radio network nodes. Typically, each radio network node transmits its own unique signals for broadcast (PBCH) and sync channels (PSS, SSS).
The concept of a point (e.g., a radio network node) is often used in conjunction with techniques for coordinated multipoint (CoMP). In this context, a point corresponds to a set of antennas covering essentially the same geographical area in a similar manner. Thus a point might correspond to one of the sectors at a site, but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or having antenna diagrams pointing in sufficiently different directions. Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point from a scheduling point of view is operated more or less independently from the other points.
The mentioned classical strategy of one cell-id per point is depicted in FIG. 7 for a heterogeneous deployment where a number of low power (pico) points 112 are placed within the coverage area of a higher power macro point 102, and provide service to various UEs 700. Note that similar principles may also apply to classical macro-cellular deployments where all points have similar output power and perhaps placed in a more regular fashion than what is the case for a heterogeneous deployment.
An alternative to the classical deployment strategy is to instead let all the UEs within the geographical area outlined by the coverage of the high power macro point 102 be served with signals associated with the same cell-id. In other words, from a UE perspective, the received signals appear coming from a single cell. This is illustrated in FIG. 8, which illustrates a heterogeneous radio communications network with a same cell-identifier assigned to each point (e.g., radio network node). Note that only one macro point 102 is shown, other macro points would typically use different cell-ids (corresponding to different cells) unless they are co-located at the same site (corresponding to other sectors of the macro site). In the latter case of several co-located macro points, the same cell-id may be shared across the co-located macro-points and those pico points 112 that correspond to the union of the coverage areas of the macro points. Sync, BCH and control channels are all transmitted from the high power point 102 while data can be transmitted to a UE 700 also from low power points 112 by using shared data transmissions (PDSCH) relying on UE specific RS. Such an approach has benefits for those UEs 700 that are capable of PDSCH based on UE specific RS while UEs 700 only supporting CRS for PDSCH (which is likely to at least include all Release 8/9 UEs for FDD) have to settle with the transmission from the high power point and thus will not benefit in the downlink from the deployment of extra low power points.
The single cell-id approach is geared towards situations in which there is fast backhaul communication between the points associated to the same cell. A typical case would be a base station serving one or more sectors on a macro level as well as having fast fiber connections to remote radio units (RRUs) playing the role of the other points sharing the same cell-id. Those RRUs could represent low power points with one or more antennas each. Another example is when all the points have a similar power class with no single point having more significance in than the others. The base station would then handle the signals from all RRUs in a similar manner.
A clear advantage of the shared cell approach compared with the classical one is that the typically involved handover procedure between cells only needs to be invoked on a macro basis. Another important advantage is that interference from CRS is greatly reduced since CRS does not have to be transmitted from every point. There is also much greater flexibility in coordination and scheduling among the points which means the network can avoid relying on the inflexible concept of semi-statically configured “low interference” subframes, as illustrated in FIG. 6. A shared cell approach also allows decoupling of the downlink with the uplink so that for example path loss based reception point selection can be performed in uplink while not creating a severe interference problem for the downlink, where the UE 700 may be served by a transmission point different from the point used in the uplink reception. Typically, this means that the UE's 700 transmissions are received by a pico point 112, while in downlink the UE 700 receives from the macro point 102.
Uplink Power Control in LTE
According to Rel-10 LTE, UL power control (PC) is performed by estimating a path loss (PL) term and by combining it with various UE and cell-specific power offset terms. An example PC formula from Rel-10 is shown in Equation 1, below:P=min(Pmax, 10log10(M+P0+α*PL+C)) [dBm],   (1)
where Pmax represents a cap on the output power (in dBm), M represents the scheduled UL bandwidth, P0 is a UE and/or cell-specific power offset, α is a cell-specific fractional path loss compensation factor, PL is an estimate of the path loss performed by the UE.
C is a correction term set by the closed-loop power control algorithm, which measures the received power at the base station and compares it to a target power (i.e. P0). Based on that, the base station sends explicit power control commands which instruct the UE to update the value of the term C. The update can be done either through accumulative steps, which are added to the current value of C, or by setting C to an absolute new value. The different possible accumulative steps are [−1, 0, 1, 3] dB and the absolute values are [−4, −1, 4, 1] dB. The power control commands are sent through the downlink control channel (PDCCH), either through scheduling grants or through two dedicated DCI formats (3, 3A) if the UE doesn't transmit data on the uplink.
A UE can regulate its transmit power based on the CRSs and a reference power level transmitted by a macro node, which means that in case the UE is served by a macro node in the downlink and a pico node in the uplink, the RSRP measurement that determines transmit power will not take the received power at the pico node into account. This implies that the UE can transmit with a power level that makes the received power in the pico node be far above what is determined by the macro node. The network may then employ so called closed loop power control to steer the UE's output power to a value that it sees fit.
The difference between the actual and the desired transmit power is thus dependent on the UE position and the transmission power levels of the different nodes in the heterogeneous network. As a consequence, the closed-loop power control algorithm may need a large number of accumulative power-control commands in order to adjust UE transmission power to the desired level. In case of mobility, the number of power control messages implies an undesirable increase of the signaling overhead in the downlink on the possibly saturated PDCCH resources. Furthermore, in case of mobility, the closed loop power control mechanism might not be able to track the desired power control level with sufficient accuracy.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.